header {
  width: 1000px;
  height: 44px;
  display: flex;
  justify-content: space-around;
  .logo {
    width: 60px;
    height: 44px;
    background-color: #eee;
    img {
      width: 100%;
      height: 100%;
    }
  }
  .menu {
    display: flex;
    justify-content: space-around;
    position: relative;
    & > * {
      // .menu 下的全部子标签
      list-style-type: none;
    }
    li {
      font-size: 20px;
      flex: 1 0 100px;
      &:hover {
        color: aqua;
        &::before {
          content: "";
          width: 0;
          height: 0;
          border: 10px solid transparent;
          border-top-color: black;
          display: block;
          position: absolute;
          top: -10px;
          left: -30%;
        }
      }
      &:first-child {
        color: aqua;
        text-decoration: underline;
      }
      & + li + li {
        // 匹配第二个li标签后的兄弟li标签
        background-color: pink;
      }
    }
  }
}

